Effectiveness and safety of olopatadine hydrochloride were studied in 30 patients with cedar pollenosis by administering the agent before and after cedar pollen began to disperse. After receiving a pre-scription for olopatadine hydrochloride 5mg twice a day, patients recorded nasal symptoms, any hindrance in daily activities, itching of the eye and compliance daily from the first day of treatment to the eighth week after pollen began to disperse. Patients were divided into two groups; one group of patients started the agent before pollen began to disperse (the pre-dispersion group, n=15) and the other group of patients started after pollen began to disperse (the post-dispersion group, n=15). Symptoms and findings were compared between the two groups. All scores for runny nose, nasal obstruction, severity of hindrance in daily activities, itching of the eyes, severity of symptoms and symptom-medication were lower in the pre-dispersion group than in the post-dispersion group. In the post-dispersion group, the minimol sectional area of the nasal cavity and the capacity of the nasal cavity tended to increase following administration of olopatadine hydrochloride. Olopatadine hydrochloride could be sufficiently effective for cedar pollenosis, even when administration started after pollen began to disperse. However, it seemed that when administration started before dispersion, the onset of nasal symptoms was delayed and exacerbation of symptoms was decreased. In addition, the agent seemed to be more effective for relief of itching eyes and preventing hindrance in daily activities.